Final answer:
The order of base-pair strength in DNA/RNA is GC (strongest) and AT/AU (weakest), based on the number of hydrogen bonds.
Step-by-step explanation:
The correct order of base-pair strength in DNA/RNA, from strongest to weakest, is guanine-cytosine (GC) followed by adenine-thymine (AT) or adenine-uracil (AU) in the case of RNA. The strength of base pairs is determined by the number of hydrogen bonds between them. GC base pairs are stronger because they form three hydrogen bonds, while AT base pairs (or AU in RNA) form only two hydrogen bonds. Thus the order reflects the stability provided by these hydrogen bonds, with more bonds resulting in a stronger, and therefore more stable, base pair.